1. Field of the Invention
The invention relates to an arrangement with a clamp of metal constructed as a pipe piece, and with at least one clamping screw for fastening an electric conductor in the clamp, wherein the clamp has in its wall at least one tnroughhole provided with a thread for receiving the clamping screw, which in its construction has a circumferential intended breaking point for limiting the force which is exerted on the clamping screw by means of a tool acting on the clamping screw and effecting a rotation about its axis (EP 0 750 723 B1).
2. Description of Related Art
Such a clamping screw used in such an arrangement, also called “tear-off” or “shearing-off screw”—in the following called “screw” for short—is used, for example, in high voltage technology for connecting the electrical conductors of two high voltage cables. For achieving and maintaining an electrically well conducting contact, a fixed connection between the conductors of two cables and a pipe-shaped clamp of metal is required into which the conductors are inserted. Such a clamp advantageously is composed of an aluminum alloy. The fixed connection could be ensured by using a so called torque wrench which, when tightening the screw, “rotates through” when a sufficiently tight seat of the screw has been reached. However, such a torque wrench is frequently not available on a construction site. Therefore, the screw is as a rule tightened by means of another tool, for example, a simple wrench.
In order to ensure, and also in order to be able to monitor whether the screw has been tightened sufficiently tight, the screw can be equipped as a shearing-off screw with at least one intended breaking point which, in its axial extension, is arranged as a circumferential groove. The upper part of the screw is sheared off when the strength of its intended breaking point has been reached or exceeded. The subsequently missing screw head constitutes an indication that the screw has been tightened sufficiently tight. After the screw head has been sheared off the screw does not project, or only insignificantly projects, out of the clamp. Therefore, an insulating element to be mounted over the clamp is not mechanically damaged, nor are its insulating properties impaired. However, the intended breaking point interrupts the thread of the screw, so that its load-bearing capacity is weakened. This has a greater unfavorable effect the more such intended breaking points are present in a screw. Accordingly, a universal use of such breaking points for conductors of different diameters is only possible to a limited extent. In addition, it can frequently not be avoided that parts of the sheared-off screw protrude out of the clamp in a harmful manner.
EP 1 626 187 A1 describes a tear-off screw which is constructed as a cylindrical threaded pin with a conical bore. Arranged in the bore is a thrust element constructed as a simple cylindrical pin. The tear-off screw has over its entire length an external thread by means of which it is screwed into a threaded bore of a clamping body and onto which a cap screw can be screwed. In the mounted position, the cap screw rests against the thrust element protruding from the bore of the tear-off screw. By rotating the cap screw by means of a tool, the tear-off screw, interacting with the thrust element, is torn off along a tear-off line when it rests sufficiently tight against conductor strands arranged in the clamping body.
EP 2 226 899 A1 discloses a device for connecting two electrical conductors which includes a metal clamp constructed as a pipe piece in whose wall are arranged at least two threaded holes. For securing electrical conductors in the clamp, clamping screws are screwed in the clamp in the assembly position in the threaded holes which rest with their end faces tightly against the conductors. In the mounted position, a fixing screw is arranged in another threaded hole of the clamp, which with a wedge tip rests against the end faces of the conductors inserted into the clamp. The tip is screwed into the fixing screw by means of a thread directed opposite the external thread of the fixing screw.
The screw according to the above-mentioned EP 0 750 723 B1 has along its extension a plurality of intended breaking points constructed as circumferential grooves in the threaded part of the screw, which are arranged parallel to each other and with an axial distance relative to each other. The screw is additionally equipped with a central blind-end hole having a polygonal cross section which extends into the vicinity of the intended breaking point located closest to the screw tip. For actuating the screw, a body of rotation is provided which consists of a pin with polygonal cross section intended for insertion into the blind-end hole and with a hood completely surrounding the pin at a distance, wherein the pin is fastened to the hood and which has on the outside a hexagonal nut for placing a tool against. Because of the larger number of intended breaking points, the deficiency of the reduced load-bearing capacity already described above, applies to an even greater extent. In addition, this screw can also only be used for securing conductors with predetermined dimensions if it is the object to prevent portions of the sheared-off screw from protruding in a harmful manner from the clamp.